The invention relates to brake equalizer apparatus for equalizing fluid pressure and damping shock waves or pressure surges in fluid lines of a fluid pressure-actuated system, such as a hydraulic brake system.
Devices (sometimes referred to as xe2x80x9cbrake equalizersxe2x80x9d) have been used to equalize fluid pressure and damp shock waves and pressure surges, in fluid lines of hydraulic brake systems. Brake equalizers are described in U.S. Pat. No. 4,166,655, issued Sep. 4, 1979 to the present inventor, U.S. Pat. No. 4,220,376, issued Sep. 2, 1980 to the present inventor, and U.S. Pat. No. 5,078,455, issued Jan. 7, 1992 to Washington, for example.
The brake equalizer described in U.S. Pat. No. 4,220,376 includes a rigid piston 48 attached to an elastomeric shock absorber 50. Piston 48 and shock absorber 50 are enclosed within a bore in a rigid housing 56. One end of piston 48 is exposed to fluid within chamber 42, and the other end of piston 48 abuts a first end of shock absorber 50. The other end of absorber 50 rests against a closed end of the bore. A volume between the sides of absorber 50 and housing 56 is partially filled with liquid (such as oil). An O-ring seal 84 between piston 48 and housing 56 prevents this liquid from escaping into chamber 42, and prevents fluid in chamber 42 from reaching absorber 50. Absorber 50 is said to be preferably molded from a rubber-like elastomer, such as neoprene having a Shore hardness of about 65.
The apparatus opposes pressure surges in the fluid in chamber 42 as follows. In response to each pressure increase in chamber 42, piston 48 effectively increases the volume of chamber 42 by sliding toward absorber 50, thereby compressing absorber 50 against the closed end of the bore. As absorber 50 deforms, it absorbs the energy imparted to it by moving piston 48.
When the fluid pressure drops in chamber 42, piston 48 opposes the pressure decrease by effectively decreasing the volume of chamber 42 as it slides away from absorber 50.
U.S. Pat. No. 4,166,655 discloses an improved version of the brake equalizer of U.S. Pat. No. 4,220,376. The apparatus of U.S. Pat. No. 4,166,655 differs from that of U.S. Pat. No. 4,220,376 primarily by including a diaphragm 18 between the piston (piston 16) and the chamber within which fluid pressure variations are to be equalized (chamber 60). Diaphragm 18 is comprised of resilient elastomer, and preferably includes a metal plate bonded to the elastomer to protect the elastomer from damage by contaminants in flowing fluid in chamber 60. The function of diaphragm 18 is to seal the piston/shock absorber assembly from fluid in chamber 60, while transmitting pressure variations in such fluid to the piston.
Also, the U.S. Pat. No. 4,166,655 apparatus includes annular rubber shims (98, 100) around the piston, in addition to an O-ring seal 96, for the stated purpose of improving the fluid seal between the piston and the housing against which the piston slides. U.S. Pat. No. 4,166,655 teaches that the housing (including body 20 and cap 22) has a preferred length of 6.4 inches, and that absorber 14 is preferably molded of neoprene having a Shore hardness in the range between 62 and 65.
The brake equalizer of U.S. Pat. No. 5,078,455 is similar in relevant respects to that of U.S. Pat. No. 4,166,655. The piston of U.S. Pat. No. 5,078,455 has two O-ring seals 12 around it (in counterbores 92 and 93) rather than one, for providing a fluid seal between the piston and the surrounding cylindrical housing surface.
The inventor of the present application has recognized that conventional brake equalizers have several limitations, including the following:
1. they have slow reaction time, and thus are unable to equalize rapid pressure fluctuations of the type which often occur in brake air line systems; and
2. they have large overall length and weight, which renders them impractical to use in certain applications.
The invention is an improved brake equalizer apparatus for equalizing fluid pressure and damping shock waves or pressure surges in fluid lines of a fluid pressure-actuated system, such as a hydraulic brake system. The inventive apparatus has a shorter reaction time than a conventional brake equalizer, and can be manufactured with smaller overall length and weight.
An important feature of the invention which results in reduced brake equalizer reaction time, is an improved design for a brake equalizer piston. The inventive piston is shorter than a conventional brake equalizer piston (the ratio of the inventive piston""s length to the length of the shock absorber which it abuts is substantially less than 0.5, and is preferably equal to about 0.23). The inventive piston is also lighter than a conventional brake equalizer piston, and is preferably made of a hard, non-compressible, material having a low-friction surface (preferably a hard, castable, self-lubricating material such as xe2x80x9cRoyal Castxe2x80x9d  ROYALCAST(copyright) urethane, which has a hardness of 79 D on the Rockwell hardness scale).
The inventive brake equalizer does not employ any seal (such as an O-ring seal) between the piston and the surrounding housing along which the piston slides, thus eliminating friction that would otherwise be present between such a seal and the housing as the piston slides along the housing. Besides reducing the reaction time of the brake equalizer (by reducing the piston assembly""s sliding friction), elimination of seals from around the inventive piston also eliminates the need for providing oil in the volume between the sides of the piston/shock absorber assembly and the surrounding housing.
In a preferred embodiment, the inventive piston is molded of xe2x80x9cRoyal Castxe2x80x9d  ROYALCAST(copyright) urethane and has a weight of about 3.5 ounces. The inventive piston is preferably used with, but not bonded to, a shock absorber. The shock absorber is preferably made from a resilient material harder than conventional brake equalizer absorber material. Preferably, the shock absorber has a Shore hardness in the range of about 70 to about 75, and is made of a combination of polyether-based urethane (cured with a liquid diamine) and a quantity of a benzoate ester (rather than BUNA-N hard rubber as in the prior art).
The inventive piston design allows the overall length of inventive brake equalizer to be reduced. The inventive piston design enables a preferred embodiment of the brake equalizer""s housing (including body and cap portions) to have a length of 4.2 inches or even less (in contrast with the housing of U.S. Pat. No. 4,166,655 which has a preferred length of 6.4 inches).
The inventive brake equalizer design eliminates the need for a metal plate to be bonded to the elastic diaphragm which separates the piston from the variable-pressure fluid (whose pressure is to be equalized). A preferred embodiment of the inventive brake equalizer includes a screen positioned between the diaphragm and the variable-pressure fluid, to prevent contaminants (especially sharp or abrasive particles) from reaching the diaphragm and possibly penetrating through it to the piston/absorber.